The lysomal enzyme .alpha.-L-iduronidase (IDUA; glycosaminoglycan .alpha.-L-iduronohydrolase, EC 3.2.1.76) hydrolyzes the nonreducing terminal .alpha.-L-iduronide glycosidic bonds in the glycosaminoglycans heparan sulfate and dermatan sulfate (1,2). IDUA has served as a model for process and maturation events undergone by lysosomal enzymes (3-8). A deficiency of IDUA in humans results in the lysosomal storage disorder mucopolysaccharidosis type I (MPS-I; cp-onyms, Hurler, Hurler/Scheic, and Scheic syndromes), which is inherited as an autosomal recessive disease and show wide variation of clinical presentation. Severely affected patients have mental retardation, somatic tissue complications and a reduced life span, while mildly affected patients may have only mild somatic complications and a normal life span. Multiple different mutant alleles at the IDUA locus are thought to be responsible for the spectrum of clinical phenotypes (1,9), but biochemical characterisation of the residual IDUA activity has enabled discrimination only between the extremes of clinical phenotypes (10-12). In work leading up to the present invention, the isolation of the IDUA gene was undertaken to provide a DNA probe for molecular analysis of mutations in MPS-I patients and for use in enzyme and gene therapy experiments in the canine model (1,3) of MPS-I.